The present invention relates to organic thin-film structures and, more particularly, to thin-film structures such as organic thin-film light-emitting diodes and systems for the fabrication thereof.
Organic light-emitting diodes are of considerable interest in being used to form pixels in electronically operated flat panel electroluminescent displays. In comparison with backlit active-matrix liquid crystal based flat panel displays, such organic light-emitting diode based displays offer a greater viewing angle, more rapid responses to control signals and less power dissipation.
Such diodes are formed of two closely spaced electrodes of which at least one is transparent to visible light. In the narrow space between these two electrodes there is provided one, or more layers of, luminescent organic materials so that, when an electric current is established between the electrodes, such a layer emits light of a color depending on the particular organic material used. Thus, an organic material layer might be chosen to be of polythiopene for providing a red light, or of polyphenylenvinylen for green light, or of polyfluorene for blue light. The transparent electrode can serve as the flat panel substrate for fabricating the organic thin-film light-emitting diodes thereon, and is typically formed of glass with indium tin oxide deposited thereon to form an anode. The remaining electrode, the cathode, is a metal system such as magnesium-silver alloy, or lithium-aluminum alloy, or calcium.
In large quantity production of relatively large area organic light-emitting diode based flat panel displays, new manufacturing methodologies are needed to provide thin-films on the glass substrate that are uniform in deposited material characteristics, and in thickness, despite being formed at high rates over relatively large areas. One of the key techniques being used to form such films is evaporation film deposition where the material species of interest to be evaporated is heated to produce vapors thereof to form a flux of molecules along a desired path to the substrate for deposition thereon. This process takes place in a vacuum system which typically comprises a deposition chamber, some means therein on which to mount the substrate panel for selected material depositions thereon, a pump to evacuate the system, pressure gauges, and one or more material evaporation sources. Production of a uniform thin-film coating on the substrate surface, or on the surface of a previous layer coating the substrate or another previous layer, often requires rotation of the substrate panel to average the deposited flux over the substrate surface, or over the surface of a layer previously deposited on the substrate or another layer, to thereby reduce or eliminate any deposition nonuniformities resulting from flux distribution time or spatial variations.
There are a wide variety of material evaporation sources available and in previous use. The most common type of such evaporators utilizes thermal heating of the material species therein that is selected for deposition to produce a resulting vapor of that material, the evaporant. This material is initially provided inside a container, or crucible, in the evaporator which crucible is surrounded by closely adjacent heating elements. Layers of thermal shielding are provided about these heating elements and the crucible to thereby confine much of the heat generated to thereby cause it to diffuse into the crucible, the crucible having an opening therein through which the evaporant is allowed to escape to form the flux thereof. The spatial variation of the evaporant in the region outside this opening depends on the angle between the spatial location of interest and the center line of the crucible opening normal to the plane of the opening, and follows approximately a cosine function of that angle. Due to this approximate cosine distribution of the deposited evaporant flux, uniformity of the deposition can be improved by rotating the substrate about an axis more or less along the flux path. For relatively large area substrates, this rotation thereof in the deposition chamber becomes increasingly more difficult with increasing size, and the means for providing such rotation in the evacuated deposition chamber adds considerable complexity to the design and operation thereof. Thus, a material evaporator source which does not require rotation of deposition substrates therein is highly desired because such an avoidance reduces the complexity and cost of system manufacture and extends the mean time between failures of the system in operational use due to the resulting reduced number of moving parts to thereby reduce operating costs. However, as the size of substrate panels increases to thereby increase the resulting display viewing area, deposited thin-film uniformity becomes increasingly more difficult to achieve, especially without rotation of the substrates. Although better uniformity can be accomplished using multiple flux sources in the evaporator, consistent control of all such sources concurrently is difficult to achieve thereby making the desired film uniformity also difficult to achieve.
One variation used in material evaporation sources has been the addition of a vapor transport section which may be separated from the crucible by a valve mechanism. To prevent vapor condensation in this section, and also sometimes to change the chemical or physical nature of the evaporant, such a section is usually independently heated. Such a material evaporation source is more versatile because the valve mechanism allows fine adjustment of the flux to maintain stability and conserve material. However, the flux distribution reaching the substrate in the deposition chamber follows a similar approximate cosine distribution.
More recently in another material evaporation source variation, a gas distribution manifold has been added to the above described heated vapor transport section. This manifold is placed so as to be directly exposed to the surface of the substrate upon which depositions are to occur such that the evaporants are discharged and deposited on that substrate surface. This manifold may also serve as a means where two or more gas phase material species may be mixed before being discharged for deposition. Again a valve means is employed to allow controlling the amount of material discharged, or the flux. Also, again, however, the flux distribution after emission from a manifold opening to reach the substrate in the deposition chamber follows a similar approximate cosine distribution. Thus, there is a desire for a material evaporation source that can deposit a relatively uniform thin-film on a surface of a substrate, or on the surface of a film previously deposited on a substrate, without the need for rotating that substrate during depositions of films.